1. Field of the Invention
The present invention relates to semiconductor metrology and more particularly to measuring properties of a transparent film on a substrate.
2. Description of Related Art
In a semiconductor manufacturing process of a substrate, a photo-resist layer is applied to a top surface of the substrate. Selected portions of the photo-resist may be exposed to electromagnetic radiation through a lithographic mask and ultimately removed. Ideally, the walls of the photo-resist material remaining after the exposure are smooth and about vertical. However, during exposure of the photo-resist, the light is reflected from the boundary between the photo-resist layer and the surface of the substrate. The reflected light interferes with the incident light in the photo-resist layer causing interference patterns and standing waves. Near the edge of a mask pattern, the interference patterns and standing waves can cause an uneven vertical wall in the photo-resist material after the exposed photo-resist material has been removed. Moreover, features below the surface of the substrate can add additional standing wave patterns creating additional edge roughness in the walls of the photo-resist.
An anti-reflective layer (ARL) may be deposited on the surface of the substrate (between the substrate and the photo-resist layer) to minimize reflections of the light from the surface of the substrate and suppress reflected light from below the substrate surface. The ARL can improve smoothness of the walls of the photo resist.
The ARL properties include, but are not limited to, thickness, reflectivity, refractive index, and extinction coefficient. These properties can be selected and/or adjusted to minimize the reflectivity of the substrate at the wavelength used for exposing the photo-resist. Measurements of the properties of the ARL may be used to adjust and optimize the ARL for suppressing the effects of reflected light at the wavelength used to expose the photo-resist.
One method of measuring the properties of the ARL is to cut (e.g., score and break) the substrate, thus exposing a cross-section of the ARL. The substrate may then be examined using light or scanning electron microscopy (SEM). Unfortunately, the number of features that can be examined may be limited to features lying along the cut. Further, although microscopy can be used to measure the thickness of an ARL, microcopy cannot measure the refractive index or the extinction coefficient. Unfortunately, the substrate is generally destroyed by cutting.
Another method for measuring properties of the ARL includes using a test substrate. A test substrate is a substrate solely used for testing one or more process tools. A substrate is a base layer, surface, semiconductor material, or non-semiconductor material upon which a feature is deposited, removed, etched, attached, or otherwise prepared or fabricated. Examples of a substrate include ceramic, plastic, glass, silicon, germanium, silicon on insulator (SOI), and gallium arsenide.
In one example, the test substrate is used to measure performance of a process tool but is otherwise not suitable for use in production. An example of a test substrate is a bare wafer onto which the ARL is deposited. A silicon wafer having a crystalline structure that is not suitable for fabricating semiconductor devices is commonly used as a test substrate. Another example of a test substrate is a lithograph mask that has been fabricated using test grade glass instead of production grade glass. Generally test substrates are less expensive than production grade substrates.
The reflectivity of the ARL is measured at a desired wavelength (e.g., the wavelength used for exposing the photo-resist). The thickness, refractive index, and/or extinction coefficient of the ARL are adjusted to minimize reflectivity at the desired wavelength (e.g., during a lithographic process). For example, the refractive index and/or the extinction coefficient of a material may be adjusted by changing the material composition of the ARL. The thickness may be adjusted by controlling the deposition time for the ARL. The ARL is then applied to another test substrate to confirm suppression of the reflected light at the desired wavelength. Several iterations may be required.
Unfortunately, the manufacturing process for the production substrates is generally stopped while running the test substrates through the process and adjusting the process. Moreover, the performance of the ARL on the test substrate may not reflect the performance of the ARL on the production substrates. Complex experiments and calculations may be necessary to match the test substrate results to the production substrate performance.
Another method for measuring properties of the ARL on the substrate includes measuring reflected light at the desired wavelength from a light beam having a small spot size directed to regions between dies on a production substrate (also known as scribe lines, streets or alleys). The scribe lines are selected because they are regions where there are no features or underlying structure between the dies.
The features in the dies can add noise to the measurements. Unfortunately, measuring the properties of the ARL using this method requires aligning the substrate to place the small spot size into the streets between the dies. Alignment requires fine focus position control and is generally difficult and/or time consuming. Moreover, the ARL in the streets and alleys may not be representative of the ARL over the dies.